Electromagnetic radiation propagates as a wave and, in an isotropic uniform medium, consists of oscillating electric and magnetic fields at right angles to one another and to the propagation direction. Electromagnetic radiation comes in discrete packets known as photons. Photons are the basic unit of light. Photons were first postulated by Planck who showed that electromagnetic radiation had to come in discrete units. Because the energy of photons is directly proportional to their frequency, low-energy photons have low frequencies, while high-energy photons have high frequencies. Low-energy photons include radio waves or microwaves, medium-energy photons include visible light, high-energy photons include X-rays, while those having higher energy still are called gamma rays.
LASER is an acronym for “light amplification by stimulated emission of radiation.” The first working laser was built in 1960 and made use of optical pumping of a ruby crystal from a flash lamp. The first continuous laser was produced in 1962 using an arc lamp instead of a flash lamp. Since its development, the laser has been used in a multitude of different applications and has become virtually ubiquitous in consumer electronics. A laser emits an electromagnetic wave having electrical field and magnetic field components. Unfortunately, the light emitted by a laser is an electromagnetic wave that cannot be localized at target regions significantly smaller than the wavelength (on order of fractions of a micron for visible light). So, while lasers are an extraordinarily valuable technology, the use of lasers is generally limited to applications in which the target regions are significantly larger than the wavelength of the laser light. This has excluded the practical use of the laser from various biotechnical applications, for example, where the target region is on the order of a few nanometers (1 nanometer (nm)=10−9 meter) and is thus 100 to 1000 times smaller than a typical laser wavelength.
There are several well-known devices and methods for channeling the energy of laser light to the nanoscale using surface plasmon resonances. One of them is apertureless NSOM (near-field scanning optical microscope), which uses a sharp metal (usually, gold or silver) tip with the radius of curvature of typically 30 to 50 nm, irradiated by an external laser light. This radiation excites surface plasmon oscillations at the tip, creating high oscillating local fields localized at the tip in nanoscale areas with sizes comparable to tip's curvature. These localized oscillating electric fields are used to probe surfaces and molecules with resolutions on the order of approximately 30-100 nm. The limitation of such devices is that the only a negligible (typically, 10−7) fraction of the laser energy is concentrated on nanoscale. It is, therefore, difficult or impossible to control properties (e.g., specific plasmon modes excited, shape of the localization region, and polarization and amplitude of the fields). It also is difficult to fabricate an effective nanometer tip—conventionally, only one in 20 tips work satisfactorily.
Another group of devices and methods have been developed to exploit surface plasmons for the sensing of chemical and biological agents. Such a device normally includes an interface between a metal and dielectric medium that possesses surface plasmon modes. These modes are excited by an external laser source to create oscillating electric fields at this interface. These fields excite molecules adsorbed at this interface, where the detection is done by either measuring absorption resonances of the excitation laser light, or by detecting Raman scattering from those adsorbates. The limitation of such methods and devices is that they generally are only usable with a comparatively large number of molecules, are incapable of detection single molecules or biological particles, and they do not have nanometer-scale spatial resolution in the lateral direction.
Therefore, there is a need in the art for the generation of an oscillating electric field on a nanoscale (i.e., on the order of 1-100 nanometers). This electric field should result from the emission of surface plasmons (electric oscillations in matter) and should be able to be localized on a target region on the order of nanometers, i.e., by a factor of thousand shorter than the practical target region for a laser.